1. Field of the Invention
This invention relates generally to interface circuits for use in telephone systems and more particularly to a trunk interface circuit between a PABX or central office (C.O.) and a two line trunk circuit.
2. Description of the Prior Art
In a telephone network, trunks are used to provide links between various telephone switching facilities. In the case of a two-wire trunk, used for example between a private branch exchange (PABX) and a central office, a PABX trunk interface circuit and a C.O. trunk interface circuit are used to provide termination for the trunk. One of the wires in the two-wire trunk is designated tip and the other is designated ring.
The trunk interface contains signaling circuitry--for performing outgoing signalling including off-hook, ring ground, and dialing, and for receiving incoming signalling including ringing, tip ground, and forward and reverse loop current; transmission circuitry--for the transmission and reception of audio signals, i.e., the circuits that carry our voices; and isolation circuitry--for electrically isolating the PABX from the central office.
Many existing systems used separate relays and detectors for each function. These designs were expensive because a separate detector circuit was provided for each function. Circuits were provided for ringing detection, tip ground detection, forward current detection, and reverse current detection. Relays were provided for signalling ring ground, off hook, and dialing.
In U.S. Pat. No. 4,361,732, by the present inventor, an improved interface circuit reduced the number of detector circuits to two by using relays to switch the detectors.
The interface circuits reflect per line expenses and, thus, any increase in the costs of an interface is multiplied by the number of lines utilized by a customer. The cost of these extra detectors and associated circuitry thus becomes a significant cost to the customer. Additionally, a system utilizing two detectors to sense loop current directions presents two output signals which must be processed, thereby further increasing the cost per line of prior art systems.